1. Field of the Invention
This invention relates to a heat-transfer sheet capable of expressing a gradation in heat sensitive, heat-transfer recording.
2. Description of the Prior Art
As heat-transfer systems, there have previously been known a heat-sublimation transfer process which comprises forming on a support an ink layer containing a heat-sublimable dye and allowing the dye to sublime by heating and transfer onto an image receiving sheet, thereby to perform the recording; and a hot-melt transfer process which comprises forming on a support a heat-meltable ink layer containing a colored dye or pigment and heating the ink layer to allow the molten ink to transfer onto an image receiving sheet, thereby to perform the recording (hereinafter a support carrying an ink layer is sometimes referred to as donor sheet). The heat-sublimation transfer process is characterized by an excellent gradation of the recorded image, because it is formed by the condensation of the vaporized dye, and the process is looked upon as promising for full color recording. Inventions relating to an improvement in the dyeability of image receiving sheets have been disclosed in Japanese Patent Application "Kokai" (Laid-open) Nos. 91,296/82, 107,885/82, 137,191/82, 49,495/84.
The heat-sublimation transfer process, however, has the disadvantage of a low recording speed owing to a high sublimation temperature of the dye which also requires prolonged heating. Although a dye which sublimes at a lower temperature may be used, such a dye produces a transferred image of low preservability due to resublimation of the dye. For this reason, various attempts have been made to improve the gradation of the image obtained by the hot-melt transfer process which has a higher recording speed and has an improved image preservability.
For instance, Japanese Patent Application "Kokai" (Laid-open) No. 56,295/82 discloses a heat-transfer sheet comprising a heat-meltable ink layer (A) provided on a support and an overcoating of a heat-meltable ink layer (B) which has a melting point lower than that of (A) which is disposed in a pattern of dots isolated from one another. This sheet reproduces a half-tone by the modulation of the amount of transferred ink by heating. Japanese Patent Application "Kokai" (Laid-open) No. 64,391/84 discloses a heat-transfer sheet comprising a support and, coated thereon successively, a layer containing an image-forming substance capable of forming an image by heating and an image-receiving layer capable of transferring the image by heating to a material which receives the transferred image, whereby the heat-transfer of the image-forming substance to the image-receiving material is controlled.
The above-mentioned two patent applications intend to reproduce the gradation by modifying the structure of ink layer coated on a support. Such a structure requires double-coating to obtain a donor sheet, resulting in an increase in manufacturing costs.
Conventional single-layer donor sheets are obtained by coating a support, by gravure or flexographic printing, with a hot-melt ink prepared by melting colored dyes or pigments, binders, waves, and other additives. Since the waxes used in a donor sheet have a large latent heat of fusion as required for a regenerator, it is difficult to specifically reproduce the gradation of the image areas corresponding to low thermal energies. As a consequence, when the heat-transfer impression is made on a plain paper or coated paper by using such a donor sheet and a heat-sensitive facsimile or thermal printer, there is obtained a high-contrast on-off binary record without a density gradation even if the printer is provided with 16 density gradients. Therefore, efforts are being made to reproduce the gradation in a complicated manner by using a number of donor sheets which differ in transfer density and making plural superimposed impressions; or by using, in addition to said superimposed impressions, the technique of dot gradation by controlling the number of dots in the printer matrix. Such a complicated printing system has the disadvantages of high cost of the donor sheets and low printing speed.